Mercury in California Rice Systems, 2017


Project Leader

Bruce Linquist, UCCE specialist, Dept. of Plant Sciences, UC Davis

The overall goal of this project is to determine whether methylmercury discharged from California rice systems poses a health risk to humans, fish, or wildlife and, if so, how this risk can be cost effectively minimized.

Mercury is naturally present in some soils of the Sacramento Valley. Under flooded conditions in California rice fields, mercury forms methylmercury and becomes more bioavailable. Objectives guiding this research project include:

  • Identify the annual cycle of methylmercury concentration and loads in the major rivers of the Sacramento Valley.
  • Determine whether methylmercury production and discharge from rice systems is higher in certain parts of the region.
  • Compare data from typical rice systems with those in the Sacramento–San Joaquin Delta.
  • Identify viable management practices to cost effectively minimize methylmercury production where risks occur.
  • Identify underlying causes where methylmercury poses a risk.

Since 2013, when this project began, three studies have been completed. These include using historical data to assess methylmercury export from rice producing areas; determining mercury budgets at the field scale; and using alternate wetting and drying (AWD) irrigation as a potential mercury management practice.

A new area of research to quantify methylmercury concentrations in grower fields commenced in the fall of 2017 and is described later in this summary.

Mercury assessment

Mercury concentration data in irrigation sources (Sacramento and Feather rivers) and agricultural drains (Colusa Basin, Sacramento Slough) collected between 1996 and 2007 were used to assess mercury contributions from rice field discharges in the Sacramento Valley.

While rice systems may be a source of mercury, loads from rice fields are small—much less than what is found in rice grown in the Sacramento–San Joaquin Delta. Peak periods of mercury concentration in drainage water exiting rice fields occur primarily during the fallow season and to a lesser extent during the early growing season. This is a consistent pattern in studies conducted in the Sacramento Valley, Cosumnes River Preserve, and the Yolo Bypass.

Sacramento Valley rice fields have lower levels of mercury in soils and irrigation water than rice fields in the Yolo Bypass and Cosumnes River Preserve. Mercury concentrations in drainage water from Sacramento Valley rice fields are lower than drainage water concentrations in the other two areas.

Research also showed that rice grain concentrations of mercury are very low—among the lowest reported in the literature and well below levels of concern for human health.

Mercury concentrations in drainage water during the growing season are lower than in the fallow season. During the growing season, most rice fields become mercury sinks. During the fallow season, most rice fields are mercury sources. That is, they export more mercury than they import through irrigation water.

AWD irrigation

A controlled, replicated experiment was conducted to determine how alternate wetting and drying (AWD) water management affects methylmercury in rice systems. This practice was found to reduce mercury concentrations in water, soil, and rice grains and could be a mitigation practice to reduce mercury if needed.

Research publications

Publication of research results is crucial, as mercury in rice systems is likely to become more of a regulatory issue. Peer reviewed papers are critical in guiding scientifically based regulation.

While rice systems may be a source of mercury, loads from rice fields are small. Furthermore, most methylmercury leaves rice fields during the winter fallow period. If necessary, the alternate wetting and drying (AWD) practice of rice irrigation during the growing season could reduce loads.

Several papers have been published on this work, including The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California in the Journal of Environmental Quality, and another in the Soil Science Society of America Journal, titled Alternate Wetting and Drying Decreases Methylmercury in Flooded Rice (Oryza sativa) Systems. A final paper has been submitted to the Journal of Environmental Quality: Methylmercury Dynamics in Upper Sacramento Valley Rice Fields with Low Background Soil Mercury Levels.

Methylmercury quantification

An important goal for 2017 research was to begin quantifying methylmercury in filtered and particulate water samples going into and out of rice fields.

The current regulatory focus on methylmercury is based on unfiltered water samples. However, much of the methylmercury is likely to be bound in suspended sediment. This methylmercury may be less bioavailable than that which is dissolved in water. From an ecosystem standpoint, dissolved methylmercury is a better indicator of ecosystem health.

With this in mind, a new area of research began evaluating in-field methylmercury concentrations from filtered samples, while also quantifying the filtered methylmercury concentrations in field flood water and tailwater. Commercial fields have been identified to determine whether higher levels of methylmercury in irrigation water result in higher concentrations in field flood water and tailwater. Two pair of rice fields in this work receive fresh irrigation water and the other pair of rice fields will receive recycled irrigation water.

Samples will be taken from six commercial rice fields during the fallow 2017-18 period and during the 2018 growing season. The fields are located in Butte, Colusa, and Yolo counties. These fields are representative of major rice growing areas, had rice grown on them the previous season, and kept straw on the flooded field during the winter.

From these fields, samples will be taken from the water inlet, the field, and the outlet to determine methylmercury concentrations and loads in filtered and unfiltered water samples. Soil samples will be taken during the flooded period each season. Grain will be sampled in each field to determine methylmercury and mercury concentrations.

The inlets and outlets are set up with monitoring equipment to measure water flow rate throughout the year. The equipment is unobtrusive, so growers will be able to manage their fields as usual.

Water samples will be collected three times during the growing season and three times during winter fallow. Samples will be collected near the beginning, middle, and end of each season.